The instability that arises where there’s a difference in velocity at the boundary between two fluids—like wind blowing across water—can create Kelvin-Helmholtz waves (KHWs), which look like a series of rolling breakers hitting the beach. KHWs are frequently observed along the outermost boundary of Earth’s magnetic field, where they presumably help transfer energy and plasma from the solar wind into our planet’s magnetosphere. Yet the conditions under which these waves form and how they evolve over time are still poorly understood.

To better characterize KHWs, Ling et al. used the Advanced Relay and Technology Mission Satellite (ARTEMIS) and Geotail satellites to make simultaneous observations of the waves from opposite sides of Earth’s magnetic tail during an instability event that occurred between 13 and 14 March 2014. They then compared their point measurements with computer simulations of the magnetosphere’s response to the solar wind conditions observed during the same period of time.

The results offer evidence that KHWs develop in Earth’s magnetic tail and that their wavelengths increase as they move tailward along the boundary layer, a finding that agrees with previously published simulations. The observations also suggest that vortices created by the instability develop at roughly the same time on the tail’s dawn and dusk sides, although slight differences between the two sets of satellite observations prevented the researchers from completely ruling out dusk-dawn asymmetry.

In addition to providing new observational evidence for the growth of KHWs in Earth’s magnetic tail, these findings provide insight into how this crucial means of interspace energy transfer evolves through both time and space. (Journal of Geophysical Research: Space Physics, https://doi.org/10.1029/2018JA025183, 2018)

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